If you can't eat the grapes, say they're sour. (Sour grapes.)
Tips
history
葡萄 is an ancient loanword brought to China via the Silk Road — it likely derives from an Iranian language. The characters are purely phonetic and have no independent meaning in this context.
culture
Xinjiang's Turpan (吐鲁番) region is China's grape capital, famous for its extremely sweet grapes due to the hot, dry climate. The expression 吃不到葡萄说葡萄酸 is the Chinese equivalent of Aesop's 'sour grapes.'
